Recently we
reported preliminary absolute cross-section data
for electron emission, doubly differential in electron energy and angle, in
collisions of 2.5 MeV/u C5+ and C4+ ions with helium
atoms.Electron energy spectra for the
case of C4+ impact exhibit broad electron-loss-to-the-continuum
(ELC) peaks centered at about 1200 eV (essentially independent of scattering angle).
These ELC features, very prominent in the forward
direction and decreasing rapidly with scattering angle, are discernable at
virtually all angles.

The width,
energy and angular dependence of the observed ELC peaks suggest that they arise
partly from the metastable 1s2s(3S) state of the projectile
ions.The Compton profile of the 1s2
(1S)ground state is relatively broad therefore leading
to a broader distribution of ELC electrons than those due to the metastable
1s2s (3S) state.

Projectiles are
prepared in the tandem Van de Graaff accelerator at KSU.
Negative carbon ions are extracted from a
sputter ion source and accelerated to a terminal at 6.0MV where they pass
through a gas cell containing N2 in which electrons are
stripped.Positive ions exiting the
cell are further accelerated, charge-state selected and directed into the
target chamber.

The target
chamber is that described by Gealy et al
A magnetically shielded chamber is flooded
with helium to a pressure of approximately 0.1mT.
Electrons are energy selected by a hemispherical electrostatic
analyzer with resolutions of 5.0% in energy and ±1.1°
in angle.
The range of electron energies and angles
surveyed is 1.0 eV to 6.0 keV, and 15°
to 160° respectively.

The present
theoretical work includes two components.
Target ionization is modeled with Continuum Distorted Wave (CDW)
calculations assuming a bare projectile with an effective charge of 5.02.
Projectile ELC is modeled with a first Born
approximation calculation assuming a projectile consisting of a mixture of 1s2
and 1s2s(3S) ions.

A weighted fit
of the theoretical calculations to the data suggests that the beam contains a
metastable fraction of approximately 10%.
The observed dependence of the ELC features on scattering angle is very
accurately reflected in the calculations.
Future work will include an independent experimental assessment of this
fraction.When this is accomplished we
will be able to present corrected absolute cross sections for ground-state C4+
impact on helium.

Figures:

Figure 1:
DDCS for 2.5 MeV/u C4+ + He collisions. Open circles are measured data,
solid line is CDW theory, solid triangles are CDW plus first Born calculation of ELC
for a projectile beam containing a 10% 1s2s(3S) metastable
component.